The onset rate of pressurization of an automobile airbag is subject to strict requirements to avoid injury to an out of position vehicle occupant. An aggressive, uncontrolled onset rate of pressurization is undesirable since excessively rapid inflation of the air bag may impact against the occupant. Essential to the concept of the present invention is the discovery that propellant grains adjacent to the igniter of the inflator are susceptible to crushing or fragmentation due to a shock wave developed by the igniter resulting in a significant increase in the onset rate of gas production.